What version of DFP do you have installed.... this setting appears to be correct:
'DefaultFileName' => $row_rstblinventory['InvImage'],
now, it is possible that the recordset is not on the correct row once the page submits to itself. That is probably why the tutorial has you add it to a hidden form element and refer to the hidden form element in the code.
That may be the solution... is the url paramater maintained through the submit so that the update recordset would be properly filtered?